Installing new tub fixtures help please

I hate to be the one who joins and immediately ask for help but I have never replaced any fixtures in a bathtub before and I know there are certain things that need to be done before attempting this job. My house was built in the 70's and the previous owner never updated or changed any fixtures in this house. I want to replace the drain, overflow, spout (which is cracked) and the hot/cold water handle. I already have the tool to remove old drain so I'm good on that and the overflow is off. I have verified that the spout is screw off and not held on by a hex. My issue is the hot/cold handle. This handle base has the tub to shower selector and the newer fixtures have this selector on the spout. Is it possible to install spout with the shower selector on the spout vice the handle? Also, there is no indication of the handle that is currently installed so I probably will not be able to install the same brand. Will this be an issue? An employee at Lowes was telling me that I need to put in an access panel behind the shower to install this. If anyone could help me I would greatly appreciate it.

This might be a dumb question but when I am putting in an access panel behind the fixture, how do I determine the right location to cut the dry wall out? I'll have to do put the access panel in my closet that is behind the shower but is there an easy way to mark the location in the closet so I can make a precise cut?

Ok, that's what my plan was just wanted to make sure that's what I was supposed to do. Last question before I start this endeavor, do I shut the water off at the street or can I shut it off at the water heater? Thank you again Terry.

You need to shut both the hot and cold off to the valve. If you have a main shutoff in the house (most places do), then that's where to do it. Note, when you've shut the water off, it's often a good idea to shut the water heater off just in case it gets partly drained - depending on the type, it could burn out if the WH isn't full.

If the heater is in the basement, it will not "partially drain". IF you kept the existing valve with the integral diverter, you COULD also use a modern spout with the diverter and EITHER one would change the flow to the shower head, but NO new trim is compatible with that style of valve body.

I just wanted to thank everyone. I was able to replace the fixtures but still have a bit of leaking (I believe from the cartridge) but I was unable to remove the cartridge and didn't want to break it. However I will be stripping if this weekend again and try to find a replacement cartridge or maybe just some O rings and such.